In drilling wells for oil and gas exploration, understanding the structure and properties of the associated geological formation provides information to aid such exploration. Measurements in a borehole are typically performed to attain this understanding. Induction tools can make accurate resistivity readings of formations downhole and are an important part of well-logging. The reading of these tools is based on the induction principle in which the transmitter produces a magnetic flux, which is picked up by the receiver. Part of the flux at the receiver is coupled directly from the transmitter and does not contain any information regarding the formation. This part of the received signal is denoted as the direct coupling signal. Another part of the signal is due to conductivity of the formation acting as a secondary transformer. This part of the signal is denoted as the formation signal.
To obtain formation measurements, it is usually desirable to reduce the ratio of the direct coupling signal to the formation signal, since the direct coupling signal can saturate and contaminate the formation signal. Traditionally, an additional antenna with opposite turns, referred to as a bucking antenna, is used to cancel out the direct portion of the total signal. Since a bucking antenna pair is used for each receiver, a measurement structure including bucking antennas results in a large number of receivers, complex tool electronics, and complexity of manufacture. Further, the usefulness of such traditional measurements may be related to the precision or quality of the information derived from such measurements.